Conventional pneumatic tourniquets typically provide an inflatable cuff which may be wrapped around a proximal portion of a patient's limb, a source of compressed gas for inflating the cuff, a pressure gauge for measuring the cuff pressure, and a pressure regulating mechanism. Typically, the cuff is wrapped around the patient's limb and inflated with compressed gas to a supra-systolic pressure as high as 650 millimetres of mercury ("mmHg") in order to stop the flow of blood into the distal portion of the limb. A surgeon is thus provided with a "bloodless field" in which surgical procedures may be performed on the limb. Maintaining a bloodless surgical field makes dissection easier, renders surgical procedures less traumatic, and generally shortens the time required to complete the surgical procedure. The pressure gauge provides the operator with an indication of cuff pressure. The pressure regulating mechanism in such conventional devices is intended to maintain the cuff pressure relatively constant.
It has been estimated that about 10,000 conventional pneumatic tourniquets are currently used in about 1,000,000 surgical procedures performed annually in North America. Regrettably, the widespread use of pneumatic (and non-pneumatic) tourniquets in surgery has been accompanied by continuing reports of limb paralysis, nerve damage and other injuries believed to be attributable to tourniquets. A survey of the literature indicates that such complications may result from many factors, including:
1. Excessive cuff pressure (which may lead to nerve compression and other damage at the cuff site).
2. Insufficient cuff pressure (which may lead to passive congestion or hemorrhagic infiltration of the nerve).
3. Excessive periods of application of an inflated tourniquet to the limb.
4. Application of the tourniquet cuff without
consideration of the local limb anatomy.
Many reported cases of preventable nerve damage, limb paralysis and other injuries are believed to have resulted from the factors listed above, the most common of which appears to be overpressurization of the cuff [see, for example: D. K. Wheeler and P. R. Lipscomb, A Safety Device for a Pneumatic Tourniquet, J. Bone Joint Surg., 45A:870, 1964; W. K. Hamilton and M. D. Sokoll, Tourniquet Paralysis, Journal of the American Medical Association, 199:37, 1967; S. J. Prevoznik, Injury from Use of Pneumatic Tourniquets, Anesthesiology, 32:177, 1970; J. M. Bruner, Time, Pressure and Temperature Factors in the Safe Use of the Tourniquet, Hand, 2:39-42, 1970; D. Fry, Inaccurate Tourniquet Gauges, Br. Med. J., 1:511, 1972; A. E. Flatt, Tourniquet Time in Hand Surgery, Arch. Surg., 104:190-192, 1972; G. Burchell and G. Stack, Exsanguination of the Arm and Hand, Hand, 5:124-126, 1973]. Unfortunately, the actual incidence of tourniquet-induced complications in surgery may not be reliably estimated because the "tourniquet paralysis syndrome" (to borrow a phrase from J. Moldaver, Tourniquet Paralysis Syndrome, Arch, Surg. 68:136-144, 1954) may be difficult to detect or may be masked by the effects of surgery, because the damage is generally transient and reversible to a large extent and because such incidents may not be consistently reported due to concern over potential legal liability. (A hospital was recently found liable for nerve injuries suffered by a patient as a result of excessive pressure applied to her arm by a tourniquet ["Hospital Liable to Patient for Tourniquet Paralysis", Citation, 38:5, Oct. 15, 1978]).
Conventional tourniquets examined by the inventor which have been linked to possible nerve injuries or paralysis associated with cuff over-pressurization have been found to have malfunctioning pressure regulating mechanisms or inherent hysteresis in the pressure regulating mechanism which permitted the cuff pressure to rise about 150-400 mmHg above the desired cuff pressure (which is typically in the 200-650 mmHg range). Similar findings have been made by other investigators [see, for example: D. L. Johnson, P. D. Neufeld and R. G. Hussey, Hazards in Single-Stage Regulation of Pressure Cuffs; J. Clin. Eng., Vol. 5, pp. 59-62, 1980.] Other tourniquets have been found to have aneroid pressure gauges which produced readings inaccurate by about 200 mmHg.
Ideally, a pneumatic tourniquet should be inflated to the minimum supra-systolic pressure required to maintain a bloodless surgical field distal to the cuff. Simultaneous maintenance of a bloodless surgical field and minimization of tourniquet cuff pressure should help to minimize the likelihood of pressure related injuries [see: R. Sanders, the Tourniquet: Instrument or Weapon?, Hand, 5:119-123, 1973; and, J. C. Adams, Standard Orthopaedic Operations, Churchill P. Livingston, New York, 1976, pp. 4-5].
Theoretically, the minimum cuff pressure required to maintain a bloodless surgical field distal to the cuff should be equal to or slightly greater than the patient's systolic blood pressure, which is the maximum blood pressure produced during each cycle of the heart. However, a patient's systolic blood pressure may continually change (particularly when surgical procedures are being performed on the patient). Thus, one practical approach would be to pressurize the cuff to a supra-systolic pressure which is known to exceed, by a reasonable safety margin, the maximum value which the patient's intra-operative systolic blood pressure might reach. One difficulty with this approach is that, because the tourniquet cuff pressure is held constant throughout the surgical procedure at a pressure selected to account for a possible rise in the patient's systolic blood pressure to a "worst case" high pressure, the cuff may, for a substantial period of time, be pressurized well above the minimum pressure required to maintain a bloodless surgical field. This is an unnecessary hazard, and may be of particular concern in the case of some patients such as infants, small children, or adults with thin limbs having little protective musculature who may be particularly susceptible to injury caused by cuff over-pressurization.
A preferred approach, which overcomes the foregoing difficulty, is to vary the cuff pressure in response to variations in the patient's intra-operative systolic blood pressure, thereby maintaining a substantially constant pressure difference between the cuff pressure and the patient's systolic blood pressure. The pressure difference is selected so that the cuff is pressurized above the patient's systolic blood pressure but near the minimum supra-systolic pressure required to maintain a bloodless surgical field.
In implementing this preferred approach, the present invention provides a pneumatic tourniquet which senses the patient's systolic blood pressure during surgical procedures and which regulates the tourniquet cuff pressure as a function of the patient's intra-operative systolic blood pressure to maintain the cuff pressure near the minimum supra-systolic pressure required to maintain a bloodless surgical field. In other words, the cuff pressure is "adapted" to the patient's systolic blood pressure so as to maintain cuff pressure approximately near the minimum pressure required to provide a bloodless surgical field.
Ideally, cuff pressure is regulated as a function of the patient's intra-operative systolic blood pressure in accordance with the "preferred approach" described above. However, if the patient's blood pressure cannot reliably be sensed with accuracy then it would be undesirable to rely upon a sensed value of systolic blood pressure as a guide to regulation of tourniquet cuff pressure.
If the sensed blood pressure is unreliable, then an alternative to "adaptive" cuff pressure regulation is to fall back to the first approach described above and to maintain the cuff pressure relatively constant near a selected pressure (for example, within about 4 mmHg of a pressure in the 200-400 mmHg range). Thus, the present invention also provides a pneumatic tourniquet capable of automatically sensing and regulating cuff pressure to maintain the cuff pressure near a selected pressure.